1. Field of the Invention
This invention relates to a process for producing molten iron by melting cold iron material such as scraps, cold pig iron, reduced iron and the like, and more particularly to a process for producing molten iron of low sulfur level economically by the use of relatively inexpensive solid carbon-containing substances. The product molten iron is suitable for use as a raw material in steel making or as a raw material for casting.
2. Prior Art
Electric furnaces are generally used in the art for producing molten iron, especially for producing molten steel from cold iron material such as scraps, cold pig iron, reduced iron and the like. However, since electric power is very costly as an energy source, there have been demands for establishment of substitutive technology which permits to melt iron-making material by the use of an energy source which is available stably at low cost. As examples of such an energy source, coal, coke and low-grade solid carbon-containing substances are conceivable.
On the other hand, cupola-furnaces have been used since old days for melting cold iron material by the use of solid carbon-containing material. Cupola is a relatively simple equipment which is suitable for small-scale production of pig iron but requires lumpy coke of appropriate sizes for maintaining gas flows in the furnace and of high strength to endure the load of charged iron source. These requirements impose great restrictions on the material to be used and also lead to an increase in production cost.
A technology aiming at solution of this problem is found, for example, in Japanese Patent Publication No. 59-44363 disclosing a method of providing a space above molten iron phase in a furnace, and charging scraps and a carbon-containing substance into the molten iron while blowing an oxygen-containing gas thereinto to combust the carbon-containing substance. This method permits to use a carbon-containing substance of any shape as it is directly charged into the molten iron and a space for combustion of CO gas is provided over the molten iron phase. With regard to the technology of enhancing the charging amount of scraps in the field of iron making by converter, there has been proposed a method in Japanese Patent Publication No. 56-8085 in which a carbon-containing substance is replenished to molten iron as a heat source while an oxygen-containing gas is supplied to urge decarbonization reaction, combusting the resulting gas on the molten iron phase.
There has also been proposed a method of combusting a carbon-containing substance by the use of a burner instead of directly charging a carbon-containing substance into molten iron as in the above-described methods. For example, the top blowing lance of a converter is arranged to have a burner construction thereby burning the carbon-containing material and heating and melting scraps from above by the combustion heat.
Of the above-mentioned conventional methods, the method of directly charging carbon-containing substance into molten iron has various demerits resulting from accumulation of ash and sulfur contents of the carbon-containing material. Namely, the ash content in carbon-containing material normally consists of acidic components, mainly SiO.sub.2, and scarcely contains basic components, so that a large amount of fluxing agents such as burned lime and dolomite has to be used for adjustment of basicity of the slag to be produced. Consequently, the operation involves an increased amount of slag, increasing the quantity of heat which is carried away with the slag and therefore raising the consumption of the carbon-containing material. Besides, the S concentration in molten iron is increased by the sulfur content in the carbon-containing material, necessitating to use a large amount of desulfurizer or to effect desulfurization outside the furnace.
The diagram of FIG. 7 shows variations in ash content in coal in relation with the amount of fluxing agents (burned lime and dolomite) and the amount of coal to be used. As seen therein, the amounts of fluxing agents and coal increase with the ash content in coal. FIG. 8 shows the relationship between the sulfur concentrations in coal and molten iron. The sulfur concentration in molten iron increases with the sulfur content in coal. Gathering from these, it is desirable to use coal with low ash and sulfur contents, which is however too costly to adopt as a practical solution means.
In case of the method in which solid carbon-containing material is completely burned in gas streams, it is necessary to provide a sufficient space for the combustion. One can easily comply with this requirement equipment-wise. However, in the method using a burner which is fueled by carbon-containing substance, difficulties are encounted in securing a sufficient space for complete combustion due to existence of scraps packed in the furnace, and combustible smoke is likely to be generated. Conversely, where the burner is located away from scraps for the purpose of achieving complete combustion, there arises the problem of inferior heat transfer efficiency.
Thus, when combusting solid carbon-containing material to melt iron-making material, it is required to enhance the combustion efficiency to attain a high utilization rate while removing the ash and sulfur contents efficiently.
The present inventors disclosed "Method and Apparatus for Blowing Solid Fuel into Electric Furnaces & Converters" in our prior application, Japanese Laid-Open Patent Application No. 62-267407.
The invention of this prior application concerns the improvement of power consumption in electric furnaces and low-cost heat compensation in converters, namely, it is restricted to an auxiliary measure of replacing part of the processing heat source by a solid fuel. In contrast, the present invention contemplates to get all the heat power necessary for melting cold iron sources solely from solid carbonaceous material, covering a different range of application.
Therefore, in the present invention it is necessary to enhance the efficiency of heat supply from the solid carbonaceous material to melt the iron source in a melting furnace in a more assured manner, restricting the air ratios in primary and secondary combustion stages.
Although the process of the above-mentioned prior application is directed specifically to electric furnaces and converters, the present invention is not restricted to the furnaces of these types.
The product of the process in the prior application is molten steel, while the product of the present invention is molten iron, especially molten iron with C concentration higher than 2%.
The present invention combines desulfurization in a precombustion vessel to solve the problems resulting from the use of solid carbonaceous material, permitting to produce molten iron of high quality. This is because the problem of S content is important in the present invention where a carbonaceous material as mentioned in above is used in a wide range (i.e., in a larger amount).
Further, we have filed a patent application (Japanese Laid-Open Patent Application No. 63-28818 for "Method and Apparatus for Blowing Fuel into Electric Furnaces & Converters". In this prior application, the fuel is burned completely outside a furnace, and the spent gas is blown against the raw material in the electric furnace or converter, without the concept of primary and secondary combustions as in the present invention.
We have also filed a patent application (Japanese Laid-Open Patent Application No. 63-72814) for "Electric Furnace Steel Making Process". As the title implies, this application is directed to a process by electric furnace into which combustion exhaust gas is blown similarly to the just-mentioned prior application, likewise without the concept of primary and secondary combustions as in the present invention.